I do have a Time Machine backup, and tried first to restore the entire folder, but it said some of the items had permissions issues. With a quarter million images, there's no realistic way to check them all, so then I wondered whether just restoring the catalog.pmdb file might work. At this point, it seems like maybe it has, but I'm waiting to let all the backed-up tasks run and then do a sync and see what I get. Will report back.
BTW, is there any realistic limit for number of images in a single catalog?